This invention relates to a process for preparing a polymeric powder. In one aspect, this invention relates to the preparation of free flowing rubber particles having an average particle size within the range of about 100 microns and about 1000 microns.
Providing free flowing small particles of rubbery polymers provides challenges due to the inherently tacky nature of the polymers. Tire rubber, for example, are commonly commercially available in bales, but there are many advantages to supplying rubbery polymers in the form of free flowing particles. Free flowing rubber polymer particles are easier to handle and may be further processed without the need to grind the bales of rubber into workable size particles. Standard polymer blending, mixing and extruding equipment may be used to process free flowing particles of rubbery elastomers whereas additional equipment is required to process bales of rubber.
It is further desirable, in many applications, that the free flowing particles of rubbery polymers also be of a small particle size, i.e. a free flowing powder. Mixing of solid phase rubbery polymers into difficult to blend compositions is greatly enhanced by initially having rubbery polymers in the form of a free flowing powder. Blending of rubbery polymers into bitumens, for example, is very difficult, and a finely divided powder would disperse to smaller resin domains with less energy required for mixing. Blending of rubbery polymers into viscous polymer melts can also be greatly enhanced by the rubbery polymer particle initially being of small size. When blending rubbery particles into reactive compositions, a small rubbery polymer particle size can also be very important to prevent excessive gel formation due to the reactive material cross-linking the rubbery particles in regions where the rubbery material predominates.
Fine powders of polymers may be produced in a number of ways. One method involves injecting a high velocity jet of atomized molten polymer into a quenching fluid stream which rapidly cools and freezes the polymer into a fine powder. As the droplets of polymer freeze in this process, they are tacky, and will agglomerate when they impact each other. The size of the particles which can be produced by this process is therefore limited to larger sizes than are desired for some applications. Fouling can also occur in process equipment due to the tacky particles impacting the equipment. This process is particularly undesirable when applied to polymers which are not produced as a melt. The step of melting the polymers is expensive, requires additional capital equipment and can degrade a temperature sensitive polymer.
Methods for the preparation of rubbery polymers in the form of free flowing particles are known and have been described, for example, in U.S. Pat. Nos. 4,374,941 and 4,375,497. These methods involve mixing an aqueous carbon black dispersion with a rubber latex, coagulating the mixture with a single- or two-step coagulating process, followed by coating the coagulated suspension with a coating resin. The coating resin can be a surfactant-containing mixture of a styrene/butadiene resin and a styrene-.alpha.-methylstyrene or polystyrene resin coagulated at a temperature in the range of from 70.degree.-90.degree. C. Although the rubber particles thus prepared are claimed to be tack-free and pourable, the process by which these rubber particles are prepared is unattractive because the many steps involved. Further, many polymer applications cannot tolerate the required levels of surfactants.
It is also known to produce small rubbery polymer particles by steam atomizing a cement comprising the polymer dissolved in a solvent. The steam will evaporate a portion of the solvent and cause the polymer in the droplets to form solid particles. The solid particles will be tacky, particularly while some solvent is still present in the cement. The particles will therefore have a tendency to agglomerate into larger particles upon contact with one another. Particles of rubbery polymers made by the prior art steam atomization processes therefore tend to have average particle sizes of about 1000 microns or larger. Particle sizes smaller than this are desirable for many applications. These particles may be made to be free flowing by mixing antiblocking agents, such as silica, into the particles produced in this manner, but the size particles which can be achieved is limited by agglomeration before the particles are mixed with the antiblocking agent.
A typical method to prepare very small size particles of rubbery polymers is to grind the polymeric particles after cooling the particles cryogenically. Although this method can result in a sufficiently fine powder, it is very expensive. An alternative process to generate small particles of polymeric powders is to grind the rubber using known technology, and then separate the ground particles, recovering the size particles which are desired. This, again, is expensive and requires additional capital equipment.
It is therefore an object of this invention to provide a process for producing polymer particles, the polymer particles being a free flowing powder. An object in a preferred embodiment of this invention provides a process to produce free flowing elastomeric thermoplastic particles having an average particle size between about 100 microns and about 1000 microns. In another preferred embodiment, it is an object of this invention to provide a process to produce polymeric particles in which the process equipment fouls at a reduced rate.